The 76 mm MK 75 MOD O/1 gun mount is fully automatic and capable of rapid fire. The gun mount includes mechanisms for hoisting cartridges vertically from the magazine, rotating the cartridges to a horizontal position and depositing them in a ramming tray, and then ramming the cartridges into the gun. In order to isolate problems and facilitate corrective maintenance when malfunctions of the gun mount occur, the 76 mm MK 168 MOD O cartridge was developed. This cartridge includes a propellant charge and a BL&P (blind loaded and plugged) projectile. That is, the projectile is loaded with an inert load and capped with a plug simulating the fuze. The load and dummy fuze are sized to duplicate the mass characteristics of an inservice projectile.
The 76 mm MK 168 MOD O cartridge just described is fully compatible with the cycling and ramming operations of the 76 mm MK 75 MOD O/1 gun mount. However, to achieve this compatability, the BL&P projectile body must separate from the propellant charge and seat in the forcing cone of the gun tube liner during ramming. Normally, the BL&P projectile is extracted by gun firing since there is no easy way of extracting a projectile through the breech once the projectile is seated in the forcing cone. This cartridge is not reuseable and requires expenditure of ammunition. Also, while this cartridge may be utilized at sea to test the cycling and ramming operations of the gun mount since the BL&P projectile could be extracted by gun firing, this cartridge obviously could not be used when the ship is in port, where better service facilities are available, due to the need to extract the projectile by gun firing.
A one piece dummy cartridge (MK 197 MOD O) has also been made and can be used to test the various cycling operations of the mount up to, but not including, the ramming operation. The one-piece dummy cartridge would rebound too rapidly during the ramming operation and interfere with movement of the sliding breech block. The ramming operation requires a unique rebound response for proper functioning and this response is not obtainable with a one-piece dummy cartridge.